• 한국전자현미경학회:학술대회논문집
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• 2005.05a
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• pp.145-146
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• 2005

• Korean Journal of Materials Research
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• v.13 no.7
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• pp.453-459
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• 2003

In this paper the effect of interstitial heat treatment on the microstructure and mechanical properties was examined both in the 0.15C-6Mn steels and 0.15C-6Mn steels added with Nb or Ti. This result will be applied into the development of a steel which has the properties of high strength and high ductility resulted from the transformation induced plasticity. The strength-elongation combination was increased as the holding time was increased when the temperature is at $625^{\circ}C$. However, the strength-elongation combination was decreased sharply as the holding time was increased when the temperature is at $675^{\circ}C$. The tensile strength and elongation of a reverse transformed steels added with Ti or Nb was 93 kg/$\textrm{mm}^2$ and 40%, respectively. This steel shows higher strength more than 10% of the 0.15C-6Mn steel without loss of ductility. The autenite formed from the reverse transformed treatment has a fine lath type, which has the width size of 0.1-0.3 $\mu\textrm{m}$. The TRIP sequence normally transforms the austenite to martensite, however, some of the sequence will produce retained austenite \longrightarrow deformation twin \longrightarrow martensite

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.10a
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• pp.273-276
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• 2008

In this study, microstructure and distribution of alloy elements were investigated in thermo-mechanically processed C-Mn-Si transformation induced plasticity (TRIP) steels. The microstructures of TRIP steels were investigated by using advanced analysis techniques, such as three dimensional atom probe tomography (3D-APT). At first, the microstructure was observed by using TEM. TEM results revealed that microstructure of C-Mn-Si TRIP steel was composed of ferrite, bainte, and retained austenite. 3D-APT was used to characterize atomic-scale partitioning of added elements at the phase interface. In the retained austenite phase, Ti and B were enriched with C. However, there was no fluctuation of Mn and Si concentration across the interface. Through these analysis techniques, the advanced characteristics of constituent microstructure in C-Mn-Si TRIP steels were identified.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.431-434
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• 2008

Effects of microstructure on the toughness of low carbon HSLA steels were investigated. Nickel decreased the ferrite-austenite transformation temperature, resulted in increase of the fraction of bainitic ferrite. However, it was decreased with increasing deformation amount at austenite region. Since fine austenite grains formed by dynamic recrystallization under large strain transformed to acicular ferrite or granular bainite rather than bainitic ferrite. The effective grain size, thus, was decreased by deformation and it resulted in lower ductile-brittle transition temperature (DBTT). The bainitic ferrite was thought to inhibit the fracture crack initiation and to delay the crack propagation by its high dislocation density and hard interlath $2^{nd}$ phase constituents, respectively. Thus, DBTT was also decreased by Ni addition in low carbon HSLA steels.

• Journal of the Korean Society for Heat Treatment
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• v.7 no.2
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• pp.88-95
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• 1994

In order to compare mechanical properties of ausformed martensite with those of marformed martemsite in Fe-30%Ni-0.1%C alloy and to investigate their strengthening mechanisms, ausformed martensite and marformed martensite were prepared by ausforming treatment and marforming treatment respectively. The microstructures were observed and the quantities of retained austenite, hardness, yield strength, ultimate tensile strength and elongation were examined. The strength of ausformed martensite was mainly increased because of the lattice defects inherited from austenite. The ductility of ausformed martensite was constant at the rate of 7-8% by ductile matrix formation of the retained austenite in spite of the increase in strength. The strength of marformed martensite was increased by the increment in dislocation density, the crossing of transformation twin with deformation twin and the mutual crossing of deformation twin. The ductility of mar formed martensite was slightly lower than that of ausformed martensite, but the strength of mar formed martensite was prominently higher.

• Journal of the Korean Society for Heat Treatment
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• v.8 no.3
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• pp.205-212
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• 1995

Effect of Mn addition on rolling contact fatigue of C-base induction hardened bearing steels has been investigated to develop inexpensive surface-hardened bearing steels with improved resistance to rolling contact fatigue. Fatigue tests were conducted in elasto-hydrodynamic lubricating conditions at a shaft speed of 5,000rpm, under max. Hertzian stress of $492kg/mm^2$. It was found in the C-Mn steels that effective depth of induction hardened layer and amount of retained austenite were slightly increased in comparison with those of C-base steels. finer interlamellar spacing of pearlite in the C-Mn steels was also observed using TEM. Decomposition of retained austenite during rolling contact fatigue was smaller in quantity in the C-Mn steels than C-base steels. This might be associated with enhanced mechanical stability of retained austenite with addition of Mn. Statistical analysis of fatigue life for C-Mn steels using Weibull distribution indicated that improved resistance to rolling contact fatigue was mainly attributed to transformation induced plasticity and mechanical stability of retained austenite.

• Journal of the Korean Society for Heat Treatment
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• v.4 no.3
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• pp.39-46
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• 1991

The effects of austenitizing temperatures and times and cooling rate on the characteristics of continuous air cooling have been investigated for 0.3%C-Mn steels microalloyed with vanadium. Transformation start temperatures have been found to be measured from temperature-time curve directly obtained with continuous air cooling and to decrease with increasing austenitizing temperature, cooling rate and Mn contents. The coarsening behavior of austenite grain size has been measured to abnormally grow at $1050^{\circ}C$ and rapidly grow at $1200^{\circ}C$. It has been found that the volume fraction of pearlite was linealy proportional to the reciprocal square root of austenite grain size. The hardness has been measured to increase with increasing cooling rate up to $250^{\circ}C/min.$ and to remain relatively unchanged in the range of $250{\sim}400^{\circ}C/min.$ showing that hardness valves for steel with a higher Mn content increase more than those for steel with a lower Mn content. The impact property has been found to decrease with increasing of austenite grain size but does not linealy change with the reciprocal square root of austenite grain size.

• Journal of the Korean Society for Heat Treatment
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• v.23 no.5
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• pp.263-268
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• 2010

This investigation was focused on the low temperature tensile properties, phase change, changes in nitrogen content and corrosion resistance in the 22Cr-5Ni-3Mo duplex stainless steel after high temperature gas nitriding and solution annealing (HTGN-SA). From the HTGN-SA treatment, the duplex (ferrite + austenite) phase changed into austenite single phase. The nitrogen content of austenite single-phase steel showed a value of ~0.54%. For the HTGN-SA treated austenitic steel, tensile strength increased with lowering test temperature, on the other hand elongation showed the maximum value of 28.2% at $-100^{\circ}C$. The strain-induced martensitic transformation gave rise to lead the maximum elongation. After HTGN-SA treatment, corrosion resistance of the austenite single-phase steel increased remarkably compared with HTGN- treated steel.

• Journal of Korea Foundry Society
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• v.19 no.1
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• pp.33-37
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• 1999

White cast iron of 3%C-10%Cr-5%Mo-5%W was casted, and then heat treated with three different methods such as homogenizing, austenitizing and tempering to observe its effects on the microstructure, hardness and abrasive wear resistance. In uni-directional soldification, bamboo tree-like $M_7C_3$ carbide grew along with the heat flow direction, and fishbone-like $M_6C$ carbide was dispersed randomly among $M_7C_3$ carbides. While almost pearlitic structures were observed in the as-cast specimen, those of the heat treated specimens consisted of secondary carbide, retained austenite and tempered martensite. In austenitized specimen, the amounts of retained austenite were 60.88% due to the higher cooling rate encountered in forced air cooling. On the other hand, the amounts of retained austenite were reduced from 60.88% to 23.85% in tempered specimen due to the transformation of austenite into tempered martensite. The hardness of tempered specimen showed the highest value, and then decreased in the order of austenitized, as-cast and homogenized specimens. But, the abrasive wear resistance of austenitized specimen was the highest, and then decreased in the order of tempered, as-cast and homogenized specimens.

• Korean Journal of Materials Research
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• v.30 no.2
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• pp.87-92
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• 2020

This study deals with the effects of austempering time on the microstructure and mechanical properties of ultra-high strength nanostructured bainitic steels with high carbon and silicon contents. The steels are composed of bainite, martensite and retained austenite by austempering and quenching. As the duration of austempering increases, the thickness of bainitic ferrite increases, but the thickness of retained austenite decreases. Some retained austenites with lower stability are more easily transformed to martensite during tensile testing, which has a detrimental effect on the elongation due to the brittleness of transformed martensite. With increasing austempering time, the hardness decreased and then remained stable because the transformation to nanostructured bainite compensates for the decrease in the volume fraction of martensite. Charpy impact test results indicated that increasing austempering time improved the impact toughness because the formation of brittle martensite was prevented by the decreased fraction and increased stability of retained austenite.